There has been an emerging need to isolate separate circuits constructed on the same integrated circuit. For example, it may be necessary to isolate a circuit with high sensitivity, such as an analog circuit, from an circuit having high transient noise, such as a digital circuit. Traditional single-ended circuits have been replaced with differential circuits in some low-frequency applications where such concerns have arisen. Differential circuits can provide better isolation in some applications as certain types of interferences may be greatly reduced. However, differential circuits are more difficult to design and test than typical single-ended circuits, particularly when such differential circuits are designed to process signals carried on radio and microwave frequencies. Traditional methods of testing differential circuits have been based on measuring voltages and currents. For example, in a typical test environment for integrated circuits, test probes engage ports on circuit wafers to determine voltages and currents between two points on a differential circuit. The use of voltages and currents fail at radio frequencies and at microwave frequencies because of difficulties in providing accurate measurements.
Scattering parameters have been developed for single-ended N-port circuits. No corresponding characterization parameters have been developed to fully characterize radio frequency and microwave frequency based differential circuits. Currently, it is possible to perform partial measurements of differential modes using a balanced probe, manufactured by CASCADE MICROTECH. However, this technique does not allow for full characterization of the differential circuit. The prior art lacks both an adequate methodology and apparatus to properly characterize radio and microwave frequency differential circuits. Therefore, a new measurement technique for characterizing radio and microwave frequency differential circuits is required.